Siderea itself is a sort of celebration of creation and frankly just fucking around. The Creators instilled the ideals of imagination and wonder into the planet they created, and part of this is seen in the commonality of shapeshifting. There is exactly one zodiac (Leo) that does not have some form of shapeshifting inherent to them, and even beyond that form-altering magic isn't exactly rare. Minor changes to one's own body are pretty much child's play, leading to the general lack of connection between sex and gender in Siderean culture.
Once you get into larger changes, like changing one's species, things get a lot more difficult and complicated. An accomplished magician (especially air signs, as shapeshifting is considered air magic) can pull it off, but you start getting out of the realm of "things any basic Siderean with basic magic knowledge" can pull off. Typically the services of experts are called on rather than people attempting to do on their own, entire "clinics" set up like the equivalent of a cosmetic surgery clinic. Of course, there's way less of a stigma against this sort of thing than on Earth: on Siderea, after all, the body is viewed as little more than an avatar through which the mind interacts with the world. It, therefore, is something that should be customized freely to best fit what's in one's soul at any given time.
The Creators intended for this, and as mentioned in post about sex and gender Sidereans have a sort of "identity radar" inherent to them. Much like a human can recognize and remember someone's face, a Siderean can recognize and remember someone's identity no matter what form they might take. This allows people to mold themselves as they please without having to deal with the struggle of trying to convince their family and friends that it's them, not being able to recognize an old friend they see in public, etc., not to mention being a lifesaver when it comes to wacky shapeshifting accidents. The effect is stronger with closeness; if you just bumped into someone on the street once and then see them in a crowd later after they've changed up their look, it wouldn't be any different than just bumping into a second stranger.
There are ways to suppress this effect for those who change their face to try and change their life. The largest factors are time and distance: if you flee your hometown and don't return for 30 years, the ability for your garbo family or w/e to recognize you would be greatly reduced if not gone altogether. Changing one's identity itself also helps: if one changes their name, moves away, and takes on a different lifestyle, they are fundamentally changing the person they are. Finally, one's identity can be obscured through things like masks, voice modulators, heavy clothing, etc.; the more a person is physically hiding their identity, the better they're metaphysically hiding their identity.
Because of this, black market dealers and other criminals will usually dress as heavily and cryptically as they can to avoid triggering the identity radars of anyone who may have known them before or who might come to know them later. There's also forbidden magic out there to suppress or alter one's inherent identity expression, but, you know. Forbidden. Suppressed as it possibly can be and heavily punished when used. If you get caught, of course. These types of magic can be quite potent for anyone who manages to figure them out, allowing them to wear their natural-born face and talk to their own mother without being perceived as anyone notable.
Another important factor, however, seems to be integral to a given person's desires and motives. Someone who doesn't want to be recognized because they're trying to escape an abuser or something like that will find that they have little trouble not being recognized even by someone who knew them for a long time. Someone who doesn't want to be recognized because they're committing a crime, however, will often end up being even more easily recognized than usual. Theory is divided between thinking the emotions of the people in question play a part in recognition - whether or not the desire to not be recognized is stronger than the desire of others to recognize - and thinking that it's an inherent part of the "programming" instilled by the Creators and based on their personal morals.
With all that being said Uhhhhhhhhhhhhh here's the shapeshifting inherent to each zodiac deets thanks
(for cusps assume everything for their component zodiacs applies to them as well)
Aries*
Ariens are capable of "winding in" their horns, a mechanic that can only be explained by some level of inherent shapeshifting magic considering there's nowhere in their skulls for said horns to go without, you know. Killing them. This is mostly just used for being able to sleep comfortably and fit through doorways.
Taurus*
Same goes for Taureans; they can just shove their horns into their noggins if they want to sleep on their side.
Gemini
The existence of the castor and pollux system makes minor shapeshifting completely trivial and greater feats much easier for Gemini. Their pollux can be shaped freely, and the ability for the castor to be dissolved and reformed allows for near-infinite possibilities. Because of the inherently high concentration of azoth (the specific cocktail of anima mundi and prima materia in Siderean bodies that allows for inherent shapeshifting) in Geminis, using magic to change their form is much easier for them than it is for other zodiacs.
Cancer
Cancers often have big, bulky carapaces, which gets in the way of a lot of things. They're able to retract their shells - the aforementioned large, bulky parts of their carapace; they'll still have carapace along their spine, etc. - as needed and alter their positioning to some extent, making them basically cool crab mech people. Typically they'll simply retract the armor enough to fit comfortable in bed or what have you rather than going full Flesh Mode, as Cancers tend to feel more comfortable when impervious to most forms of attack.
Leo
Leos are the only zodiac without some sort of inherent shapeshifting talent. Their cusps - Canceos and Lirgos - are exceptions to this, of course, but pure Leos don't really have anything going on that might significantly inconvenience them. Due to lacking any sort of primal shapeshifting instinct, they tend to struggle more with learning shapeshifting magic than other zodiacs.
Virgo
Virgos have the ability to split their legs into roots in order to give/obtain nutrients to/from the earth. Some can even just devolve their entire lower body, sometimes more! Arms are particularly common to make roots out of, though there's not any real benefit to this beyond just really liking dirt or showing off or something. Or freaking out friends, maybe. They can also reroute their internal anatomy in order to start producing different fruits or flowers.
Libra
Libras have an obvious shapeshifting mechanic in their chrysalises, but they don't need those alone. They're even more mech-esque than Cancers, having the ability to retract, open, and shift various parts of their bodies in various minor ways, typically used for storage and combat.
Scorpio
Scorpios are like Cancers to a lesser degree, as the only part of their carapace with any bulk are their pincers. Keeping them retracted is so common outside of combat that offworlders often come away with the impression that they don't have big meaty pincers hiding somewhere. Also maybe they can retract their chelicerae horn things??*
Sagittarius
Sagittarians are able to freely shift between their centaur form and a bipedal form, typically called "satyr form".
Capricorn
Like Sagittarians, Capricorns can shift between their fish goat form and a bipedal form, typically called "faun form". In faun form they may have a goat tail or fish tail, and likewise their legs might be more furry or scaly depending on their personal preference and whether they spend more time on land or in the sea. They prolly don't need horn help bc their horns tend to be very vertical but maybe they can do the horn thing too?*
Aquarius
Aquarians, basically being water vapor possessed by a soul, can shift their forms most freely of all. They can most easily shift into things that are various forms of water, and each Aquarius often has a sort of core preference for their "base" form. There's sort of air elemental mode, cloud forms, a sort of water slime thing, and of course ice time! It isn't that uncommon for Aquarians to mix together the different states of water, either. Shifting into materials that aren't water-based, however, requires magic, but it's not too hard for them to learn. Most of them don't like more "rigid" forms, though, preferring the freedom being an Aquarius offers them.
Pisces
Pisceans can shift between mer form and two-legged form, allowing them to function both underwater and on land with ease. I haven't completely decided what their legs are like yet, but they probably aren't straight-up human legs and at bare minimum prolly have some fins somewhere on there or something. They still have their mermaid tails when in bipedal form, just shifted back and shrunken a bit.
Ophiuchus
Ophiuchi, like Pisceans, can shift between snake mode and leg mode. In leg mode (official name) they pretty much just have human legs, albeit typically with scales here and there. Also like Pisceans their former noodle:tm: will still be present as a tail. Depending on age this tail can be quite unwieldy, but an Ophiuchus old enough for that to be a problem has also likely long since learned to use magic to shrink or "retract" it.
Cetus
Cetans, even more like Pisceans, can shift between mer form and two-legged form. They also find shapeshifting to be pretty simple, owing to the large amounts of anima mundi and prima materia in their bodies thanks to being Frenzy creations out of Asena juice. This is good, because it means that it's a primordial instinct sort of thing for them to be able to shift their size in order to be able to participate in normal human society. Cetans tend to be a bit apprehensive about this, though - mainly the old guard, who returned from the exodus or never left in the first place - as they find meaning in their immense forms. Not to mention it's just sort of awkward having everyone suddenly be the same size as you, or even taller, when you're used to being several stories tall...
Arachne
Araneans can shift between being a spider-taur thing and having a two-legged mode. In Leg Mode:tm: they have kinda pointy, probably kinda fuzzy legs that resemble insect legs, though they don't seem to have too much trouble walking. They'll usually still have a spider butt:tm:, albeit much smaller, serving as a sort of tail. Like Scorpios, maybe they can retract their lil chelicerae horns too.
* idk if i'm making this horn/chelicerae thing canon but it's kind of funny and on-brand for the creators really. but it might be more interesting to world-build around having species with the inconvenience of big meaty horns. but it's soooo siderea
Needless to say, all of this makes it clear why the End of Magic was such a big deal. If it was just casting spells that started being an issue then that would suck, but the planet could still carry on. But soon enough people began realizing that if prima materia was behind the functioning of their own bodies... things got messy quickly, and a large part of that is that many zodiacs wound up stuck in one form or literally falling apart when they attempted to perform extremely ordinary (for them) biological feats.